In optical data transmission, digital data may be transmitted by modulating the phase of an optical wavelength in dependence on the transmitted data values and in accordance with a constellation diagram of a respective phase-shift keying (PSK) modulation method. Each point of the constellation diagram represents a finite set of data bits that are to be transmitted. Depending on the set of data bits that are to be transmitted, the phase of the optical wavelength is changed, such that it corresponds to the respective point of the constellation diagram. Examples for phase-shift keying modulation methods are Binary Phase-Shift Keying (BPSK), in which each point of the corresponding constellation diagram represents one bit, or Quadrature Phase-Shift Keying (QPSK), in which each point of the corresponding constellation diagram represents two bits.
The set of bits represented by a point of a constellation diagram is called a symbol. The rate, by which the phase of the wavelength is changed and by which therefore symbols are transmitted, is called the symbol rate.
In order to increase the data rate for transmitting data via PSK modulation of a specific optical wavelength, a technique called Polarization Division Multiplexing (PDM) may be exploited. In PDM, for example two optical signals of a same wavelength but with respective orthogonal polarization states are modulated separately using e.g. QPSK at each signal separately, and then combined, thus forming a single optical signal which may then propagates into a fibre link.
At a receiving side, these two optical signals may be recovered from the PDM signal, by sampling the optical field resulting from the combined signal along two polarization planes that are orthogonal to each other. The sampled signals may then be used for deriving respective symbol values from them. From these symbol values, respective data values can be de-mapped.
For further increasing the data rate, not only the technique of PDM may be applied, but also the technique of Wavelength Division Multiplexing (WDM):    PDM is applied separately to different optical signals of different optical wavelengths, and then these optical signals are transmitted over a same optical fibre. On the receiving side, sampling of received optical signals along orthogonal polarization planes is carried out for each respective wavelength.
When transmitting optical signals over a non-ideal optical fibre, different effects such as cross-polarization or cross-phase modulation may cause transmission distortions, which in turn lead to signal degradation of the transmitted optical signals. Such a signal degradation may lead to errors of the transmitted data values, when deriving symbol values from sampled signals on the receiving side and de-mapping the data values from the derived symbol values.
It is a common technique to protect the transmitted data values by Forward Error Correction (FEC). Bit errors of the data values occurring on the receiving side might be compensated, by encoding the data values into a block of bits using a FEC encoding algorithm on the transmitting side, and by then decoding the received block of bits in accordance with the applied FEC algorithm on the receiving side. A FEC algorithm is able to correct only a maximum number of bit errors per FEC block.